The Canine Ledger

When to Spay or Neuter: What the Breed Studies Show, and What They Don't

Early neutering is linked to joint problems in some large breeds, not in most small ones. What the studies show, and how weak the mammary-tumour evidence is.

Written by thecanineledger

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For decades, the advice was simple: neuter every pet dog by about six months. A 2025 review in the Journal of Small Animal Practice says that advice no longer holds: “the decision to routinely neuter all pets not intended for breeding can no longer be supported for all categories of animals” (Romagnoli, J Small Anim Pract 2025).

The same author is just as clear that this is not an argument against neutering. “Gonadectomy still plays a major role in lowering the risk of mammary neoplasia and pyometra in bitches and of prostatic hypertrophy in male dogs.”

While the breed studies behind this shift are real and worth reading, they are often summarised online as “neutering causes cancer” or “never neuter before two”. Neither is what they found. This page sets out what the main studies show, where they are weak, and what to discuss with your vet.

1. Why the advice changed

Neutering (castration in males, spaying in females) removes the testes or ovaries, and with them most of the sex hormones. Those hormones also act on growing bones and other tissues.

Romagnoli summarises the shift: “a variety of non‐neoplastic and neoplastic conditions occur more commonly in spayed/neutered than entire dogs, and for several such conditions the earlier the age at surgery – the higher the risk.” He adds that “unwanted behavioural changes may occur after gonadectomy rather than expected improvement in behaviour in some dogs.”

His conclusion is that “Straightforward routine recommendations are not possible any longer”. Advice now has to take into account “age, sex, breed, animal lifestyle, purpose for which the animal is kept”, and the owner’s own circumstances.

2. The breed studies

The best-known work comes from one hospital. A team at the University of California, Davis, used the records of the university’s veterinary teaching hospital to compare dogs neutered at different ages with dogs left entire. Their 2020 paper covered 35 breeds (Hart et al., Frontiers in Veterinary Science 2020).

What they found.

  • In earlier work on Golden Retrievers, Labradors and German Shepherds, “neutering before a year of age was associated with increased risks of one or more joint disorders, 2–4 times that of intact dogs.” The joint disorders were hip dysplasia, elbow dysplasia and cruciate ligament tears.
  • In female Golden Retrievers, cancer risk was raised “with neutering at any age”.
  • “There were major breed differences in vulnerability to neutering”.
  • “Small-dog breeds seemed to have no increased risks of joint disorders associated with neutering”. Only two small breeds, the Boston Terrier and Shih Tzu, showed a significant increase in cancers.
  • For most breeds: “In most cases, the caregiver can choose the age of neutering without increasing the risks of these joint disorders or cancers.”

Mixed breeds. A companion study from the same hospital grouped mixed-breed dogs by adult weight. In dogs of 20 kg or more, neutering before one year “generally was significantly associated with risks of one or more joint disorders above that of dogs left intact, commonly to 3 times the level of intact dogs.” Under 20 kg, “no increased risks were found for joint disorders”. No weight group showed an increase in cancers (Hart et al., Frontiers in Veterinary Science 2020, abstract).

→ Cruciate Ligament Rupture: Surgery, Conservative Care, and the Evidence Gap

3. What the breed studies can’t tell you

They come from one referral hospital. A university teaching hospital sees sicker dogs, and dogs whose owners can travel and pay for specialist care. It is not a random sample of pet dogs.

They look back, not forward. These are retrospective record studies. Owners chose when to neuter, and the reasons they chose may themselves be linked to the dog’s health.

They cover a limited list of breeds and diseases. The authors name the first of their limitations: “relatively few breeds are covered compared to those included in the various breed registries”. Hart’s own conclusion also adds: “Of course, individual factors must be taken into account.”

Other work points the same way for some breeds. A 2024 Veterinary Evidence summary of eight studies concluded that spaying before one year “in Golden Retriever, German Shepherd, and Rottweiler bitches may increase the risk of cranial cruciate ligament disease. In other breeds, limited evidence is available to draw conclusions” (Low, Veterinary Evidence 2024).

A small prospective study of Labrador and Golden Retriever crossbreds bred by the UK charity Guide Dogs found cruciate disease in 11 of 155 bitches spayed before puberty, against 1 of 151 spayed after it (Moxon et al., Animals 2025).

“Before or after the first season” is mostly unstudied. A 2024 scoping review searched for studies that compared spaying before puberty with spaying after it. Out of 33 included studies, “Only six of the 33 studies categorised the timing of surgical neutering as prepubertal or post-pubertal”. It concluded that “robust evidence to support veterinarians, those working with dogs and dog owners when discussing the timing of neutering relative to puberty does not yet exist” (Moxon et al., PLOS ONE 2024).

Our note: We read “Veterinary Medical Teaching Hospital” as a referral population. We don’t print breed-by-breed ages, so ask your vet about timing for your dog.

4. The mammary tumour question

The most common reason given for spaying young is that it prevents mammary (breast) tumours.

A 2012 systematic review assessed the evidence. It found 13 relevant reports, and judged nine of them to have a high risk of bias. Its conclusion: “the evidence that neutering reduces the risk of mammary neoplasia, and the evidence that age at neutering has an effect, are judged to be weak and are not a sound basis for firm recommendations” (Beauvais et al., J Small Anim Pract 2012, abstract).

Romagnoli notes that the protective effect on mammary tumours, and the increase in urinary incontinence, “was surprisingly regarded as weak”. That is not proof that spaying doesn’t protect. It means the protection has never been measured well.

Our note: We don’t print the widely repeated “before the first season” percentages, because we haven’t read the study they come from, and the 2012 review judges the evidence weak.

→ Found a Lump? Why Your Vet Should Put a Needle in It

5. The case for spaying, and pyometra

Whatever the uncertainty about tumours, spaying lowers the risk of pyometra, a womb infection that can kill.

→ Pyometra: The Emergency in Unspayed Females

Urinary incontinence. A 2024 UK study used primary-care records to compare bitches spayed early (3 to under 7 months) with those spayed later (7 to 18 months). Later spaying gave “0.80 times the odds” of incontinence diagnosed before 8.5 years. The authors’ advice: “preference given to delaying neutering until after 7 months of age unless other major reasons justify earlier surgery” (Pegram et al., PLOS ONE 2024).

6. Male dogs, behaviour, and the alternatives

Prostate. Castration lowers the risk of benign prostate enlargement. But Romagnoli also notes that “regular prostatic checks should be planned also for surgically gonadectomised dogs due to their higher risk of developing prostatic neoplasia.”

Behaviour. Neutering is often suggested to fix behaviour problems. Romagnoli: “Gonadectomy is not an appropriate treatment for behavioural problems which should always be dealt with by a behavioural expert”. He warns that neutering “especially before puberty – may in some cases exacerbate behavioural issues”.

Our note: A reversible hormonal option before surgery has been trialled for behaviour cases. We don’t describe it. Ask your vet.

Gonad-sparing surgery. Some vets offer a hysterectomy (removing the womb but leaving the ovaries) or a vasectomy. Romagnoli says these “apparently have the same level of safety as classical spaying and neutering (although they do not eliminate reproductive behaviour).”

If you leave your dog entire, you take on the job of watching for problems. Romagnoli: these dogs “should undergo regular health checks to make sure mammary or prostatic conditions are diagnosed and treated at an early stage.” If that can’t be relied on, “it would probably be in the best interest of dogs to be gonadectomised”.

Our note: If your female dog is left entire, ask your vet what to watch for and how often she should be checked.

→ How Risky Is Anaesthesia for My Dog? · → Choosing a Breed or a Mix: The Health Data Breeders Don’t Lead With

7. What to ask your vet

  • Given my dog’s breed, or expected adult weight if mixed, is there evidence that timing matters?
  • What are the risks for my dog of waiting, and of not neutering at all?
  • If I wait, what should I watch for, and how often should we check?
  • Is a gonad-sparing operation an option here?

8. What we could not verify

Commonly saidWhat we found
“Neutering causes cancer”Not in general. The UC Davis studies found raised cancer risk in some breeds, such as female Golden Retrievers, and not in most.
“Never neuter before two years”We found no study supporting one age for all dogs. Breed and size matter.
“Spaying before the first season prevents mammary tumours”The evidence is weak, according to a 2012 systematic review.
“Small dogs should be neutered early”Not shown either way. Small breeds and dogs under 20 kg showed no rise in joint disorders, which is not the same as a benefit.
“Neutering calms a dog down”Not reliably. A 2025 review warns it may worsen some behaviour problems.
“The breed studies apply to all dogs”They don’t. They come from one hospital’s records, in a limited list of breeds.

Sources

Always confirm with your vet before acting on anything on this page.